The present invention relates to a compound and a method for the selective absorption of NO, nitrogen oxides from gaseous mixtures containing carbon dioxide.
In particular, it relates to the absorption of nitrogen oxides from the exhaust gas of internal-combustion engines.
The literature (M. Machida et al.xe2x80x94J. Chem. Soc., Chem. Commun. (1990), p. 1165, and New Frontiers in Catalysis, Proc. of the 10th Intern. Congress on Catalysis, Budapest, Hungary, Elsevier (1993) p. 2644) describes mixed barium-copper oxides which are given the formula BaCuOx, where x has the values of 2.1 and 25, and are capable of reversibly absorbing nitrogen oxides by working within a certain temperature range, fixing them as barium nitrites and nitrates, and of releasing them by heating to temperatures higher than the absorption values, restoring the structure of the initial oxides.
The above mentioned mixed oxides are highly reactive also to carbon dioxide, which they fix as highly stable barium carbonate which, by depositing on the surface of the material, inhibits its further absorbing capability.
High reactivity to carbon dioxide therefore prevents use of compounds BaCuOx to absorb nitrogen oxides from mixtures rich in carbon dioxide, such as the exhaust gas of motor vehicles.
An attempt has been made to obviate this drawback by using mixtures of BaCuO2.1/MnO2 which are scarcely sensitive to carbonatation.
Finally, it has been found that BaCuOx compounds tend to lose, over time, their capability of absorbing nitrogen oxides.
Application EP-A-666 102 describes the use of substances for adsorbing nitrogen oxides from the exhaust gas of engines designed to work with an excess of oxygen in the air/gasoline mix, capable of adsorbing NO and of converting it into NO2 by virtue of the action of the excess oxygen that is present in the mix.
When the engine runs with an oxygen deficit (air/gasoline mix rich in gasoline), the adsorbed nitrogen dioxide reacts with the reducing gases that are present in the mix (CO and unburnt hydrocarbons), becoming N2 and oxidizing the reducing gases to CO2 and H2O.
The adsorbers used in the European application are essentially constituted by mixtures of barium carbonate and copper oxide formed locally during preparation by decomposition of copper nitrate and barium acetate with Ba/Cu ratios within broad ranges (from 1:3 to 3:1).
Said adsorbers, however, are entirely inactive in fixing nitrogen oxides in the absence of oxygen or in case of oxygen deficit, such as when the engine, at startup, runs with gasoline-rich air/gasoline mixes.
Furthermore, the temperature window in which the adsorbers are active is shifted toward high temperatures, thus preventing adsorption when the engine is running cold.
WO 97/28884 discloses a compound of formula Ba2Cu3O6 suitable for adsorbing gases, among others, carbon dioxide.
U.S. Pat. No. 5,238,913 reports that compounds of formula Ba2Cu3O5+x (OL X L1) are suitable for preparing superconducting microcircuits. No indications are given about the method of preparation of the compounds and, in particular no mention is made of the compound Ba2Cu3O6.
It has now been unexpectedly found that the compound having the formula Ba2Cu3O6 and the Raman spectrum characteristics as set forth in the claims is capable of selectively absorbing nitrogen oxides NOx from gaseous mixtures rich in carbon dioxide, possibly containing pollutants such as CO, SO2, hydrocarbons and mixtures thereof. Absorption occurs at temperatures between approximately 180xc2x0 C. and 480xc2x0 C., working at atmospheric pressure.
It has furthermore been found, and it is another aspect of the invention, that nitrogen oxide absorption kinetics is accelerated considerably by the presence of water vapor in the mixtures. In the case of NO2, the presence of oxygen and moisture shifts the absorption toward relatively low temperatures comprised between approximately 180xc2x0 C. and ambient temperature. Preferably, NO2 absorption is performed at temperatures above 35xc2x0 C.-40xc2x0 C.
By effect of the absorption of considerable amounts of NOx oxides, the compound of the invention decomposes forming barium nitrite and mono- and divalent copper oxides, if they are exposed to NO in the absence of oxygen, barium nitrate and bivalent copper oxide, if they are exposed to NO2 or NO in the presence of oxygen.
The thermogravimetric curves plotted in FIGS. 1 and 2 show the absorption of NO and NO2 as a function of the temperature (absorption of mixtures of 25% NO and 3% O2 in helium, with a space velocity of 3000/h and 2.5% NO2 and 2% O2 in helium/nitrogen with a space velocity of 3000/h and a heating rate of 20xc2x0 C./min (percentages expressed by volume)).
By heating to temperatures above approximately 480xc2x0 C., the compounds that have formed begin to decompose, releasing the nitrogen oxides and restoring the Ba2Cu3O6 structure of the starting compound.
At temperatures above 480xc2x0 C., barium nitrite and nitrate and copper oxide begin to react with each other, forming the compound Ba2Cu3O6 and releasing, respectively, NO and NO2 and possibly oxygen. In the range between 480xc2x0 and 700xc2x0 C., Ba2Cu3O6 coexists alongside with barium nitrite and nitrate and with copper oxide; the Ba2Cu3O6 fraction increases with time and temperature.
The selectivity of the Ba2Cu3O6 with respect to CO2 depends considerably on the preparation method.
It has been found, and it is another aspect of the invention, that the compound of the invention considerably increases its resistance to carbonatation if it is prepared starting from barium nitrate and copper oxide intimately mixed in a cationic ratio of 2:3, subsequently heating the mixture to 640xc2x0 C.-650xc2x0 C. in an air stream until the barium nitrate is completely decomposed and then cooling the mixture in air stream at a rate of no more than 20xc2x0 C./min.
The air can be replaced with oxygen/nitrogen mixtures or oxygen/inert gas mixtures containing up to 25 g/m3 of water vapor and up to 400 ppm of CO2.
It has furthermore been found that the presence of nitrogen oxides during the cooling of the material, or their addition to the reaction atmosphere to complete the synthesis, facilitate the formation of the carbonatation-resistant materials.
The curve of carbonatation as a function of temperature which is typical of the compound Ba2Cu3O6 thus prepared as above specified is reported in FIG. 3 (stream of 10% CO2, 10% H2O, complement with mixtures of nitrogen and argon, exposure 5 hours, percentages by volume).
For comparison, the circles indicate the carbonatation behaviour of a non-resistant compound BaCuO2.5 prepared according to the methods described in literature.
The carbonatation curve of the compound supported on alumina is similar to the curve of the above mentioned compound. The preparation is made by immersing porous aluminum oxide, dehydrated beforehand, in a near-saturated solution of barium nitrate and copper nitrate in deionized water, using a barium ion/copper ion ratio of 2:3 and working at temperatures between 20xc2x0 C. and 80xc2x0 C.
The material, impregnated with the solution, is dried at 110xc2x0 C.-150xc2x0 C. and then subjected to the above described heat treatment (reaction at 640xc2x0 C.-650xc2x0 C. and then cooling at a rate of no more than 20xc2x0 C./min).
The procedure can be repeated in order to increase the filling of the pores of the aluminum oxide until saturation is reached.
Approximately 3.5% by weight of supported compound is obtained for each impregnation/heat treatment cycle.
The curve of FIG. 3 shows that the compound Ba2Cu3O6 prepared as mentioned above is not sensitive to carbonatation up to approximately 420xc2x0 C. (less than 0.4% increase in weight after 5 h of exposure). The increase is less than 2% at 500 xc2x0 C., again after 5 h of exposure.
Resistance to carbonatation decreases considerably if the compound Ba2Cu3O6 is prepared at 800xc2x0 C. and then cooled quickly to ambient temperature (rate of approximately 5xc2x0 C./sec).
Table 1 reports the weight increases by isothermal treatments in NO 1% by volume, 99% N2 of Ba2Cu3O6, in comparison with the xe2x80x9ccompoundxe2x80x9d Ba2CuO2.5 prepared according to the methods described in literature.
The table shows that the compound BaCuO2.5 ceases to absorb after approximately 12 h at temperatures between 300xc2x0 C. and 400xc2x0 C., whilst absorption continues at 500xc2x0 C. Absorption at 500xc2x0 C. is slightly more than half the absorption of Ba2Cu3O6, which instead continues to absorb prolonged periods at all temperatures from 300xc2x0 C. to 500xc2x0 C.
The Raman spectrum of the carbonatation-resistant compound Ba2Cu3O6 (prepared as herein before indicated) shown in FIG. 4 shows that the maximum intensity peak in the wave number range from 0 to 800 cmxe2x88x921 appears at wave number of 598xc2x15 cmxe2x88x921 , and that at wave number 633xc2x13 cmxe2x88x921 there is a mode whose intensity is between 0% and 30% of the intensity of the mode that appears at 598xc2x15 cmxe2x88x921, or that said mode is absent.
It is also found that at wave number 560xc2x15 cmxe2x88x921 there is a mode whose intensity is 30% less than the intensity of the mode that appears at 598xc2x15 cmxe2x88x921. A symmetric band is centered around wave number 520xc2x17 cmxe2x88x921 and has an intensity between 20% and 40% of the intensity of the mode that appears at 598xc2x15 cmxe2x88x921.
The Raman spectra were recorded with a Dilor LabRam apparatus, using a laser beam at 632.8 nm with an intensity of 1 mW, focused on sample portions measuring 1 micron in diameter.
X-ray diffraction measurements of powders and of single crystals show that the compound Ba2Cu3O6 crystalizes in the rhombic system, with cells characterized by the lattice parameters 4.18 ∈ less than a less than 4.35 xc3x85, 6.83 ∈ less than a less than 7.33 xc3x85and c=11.39xc2x10.02 xc3x85, which are the result of the distortion of a hexagonal packing in which 4.05 xc3x85 less than a less than 4.28 xc3x85, c =11.39xc2x10.02 xc3x85 and the angle xcex4 changes from 120xc2x0 to a value between 115xc2x0 and 118xc2x0.
The X-ray diffraction spectrum (powder diffraction) of the carbonatation-resistant compounds shows that the intensity of the reflections that can be detected at the angles 2 xcex8=29.7xc2x0xc3x970.05xc2x0 and 2xcex8=30.3xc2x0xc2x10.05xc2x0 is very weak and lower than 10% of the intensity of the intense reflection at 2xcex8=29.00xc2x0xc2x10.05xc2x0. The lower the intensity of these reflections, the higher the resistance to carbonatation.
The powder X-ray diffraction measurements were made using a Philips X-pert diffractometer constituted by a PW1830/40 generator, PW3719 goniometer and PW3710 control unit using Cu Kxcex1 radiation.
Advantageously, in order to increase the exposed surface area, the compounds used in the absorption method of the invention are supported on porous carriers having surface area higher than 50 m2/g preferably higher than 100 m2/g and more preferably comprised in the range of 150-500 m2/g, which are inert towards the reactants used for preparing the compounds.
Examples of said carriers are alumina, titania, zirconia, boron nitride, silicon carbide.
As mentioned, the compounds according to the invention are applied particularly in the absorption of NOx oxides from the exhaust gas of internal-combustion engines.
By virtue of the capability to absorb and desorb oxides at temperatures in the range between approximately 200xc2x0 C. and 700xc2x0 C., the compounds are used in mufflers preferably placed in a portion of the exhaust pipe which is at a temperature between approximately 200xc2x0 C. and 500xc2x0 C. when the motor is running cold and at temperatures above approximately 550xc2x0 C. when the motor is running steady.
Another application of interest of the compounds relates to the absorption of nitrogen dioxide (NO2) from the fumes of plants such as those for nitric acid and for preparing silicon.
Other applications of the compounds relate to the absorption of NOx oxides from the exhaust fumes of domestic heating systems or from fuel-burning electric power stations.
In the case of the absorption of nitrogen oxides from the exhaust fumes of fixed plants, such as heating systems or fuel-burning power stations, the compounds Ba2Cu3O5+d, once they have been converted into Ba nitrites and nitrates, can be restored to the initial fully active form by heating.
It has been found that the compounds Ba2Cu3O5+d which have already been subjected to absorption of NOx oxides and have not been fully decomposed into barium nitrate and cupric oxide oxidize hydrocarbons to CO2 and H2O and CO to CO2 even in the absence of oxygen at temperatures lower than those of pure compounds Ba2Cu3O5+d.